The present invention relates to an abnormal noise detector for finding flaws in gear units by detecting abnormal noises resulting from these flaws. More particularly, it relates to an abnormal noise detector for gear units having two or more gears which can detect abnormal noises resulting from the rotation of an eccentrically mounted gear.
A conventional method of detecting abnormal noises produced by a gear unit is to employ a microphone and a frequency analyzer. A gear unit to be tested is connected between a drive motor and a load such as a second motor and driven by the drive motor at various speeds. The noise produced by the gear unit is picked up by the microphone which produces an electrical output signal, and the electrical output signal of the microphone is then provided to the frequency analyzer as an input signal. The frequency analyzer samples the signal from the microphone for a certain period of time T and produces a sound pressure level spectrum representing the average sound pressure level of each frequency of noise produced by the gear unit during the sampling period T at the given rate of rotation. By comparing the sound pressure level spectrum for the gear unit being tested with a standard sound pressure level spectrum for a gear unit operating normally, it is possible to detect certain abnormalities in the sound level spectrum due to flaws in the gear unit being tested.
However, this method of detecting abnormal noises is not appropriate for detecting the abnormal noises produced by a gear unit with an eccentrically mounted gear. When two gears mesh, they produce noises having frequencies proportional to the rate at which the teeth of the two gears mesh. If both gears are properly mounted on their geometric centers, the teeth of the two gears will mesh at a constant rate, and the frequency of the noise produced and the sound pressure level of the noise at this frequency will be constant. However, if one of the gears is eccentrically mounted, i.e. due to either a manufacturing imperfection or a bent shaft the center of rotation of one of the gears is not the geometric center of that gear, the rate at which the teeth of the two gears mesh will not be constant, and an abnormal noise will be produced. Rather than being constant, both the frequency and the sound pressure level of the noise produced by the meshing of the gears will vary sinusoidally over time.
This phenomenon will be explained in more detail with reference to FIGS. 1 and 2 of the attached drawings. FIG. 1 is a schematic drawing of a gear unit comprising a first gear 1 and a second gear 2. The first gear has a geometric center 3 and a center of rotation 4, which is displaced from the geometric center 3 by a distance r. The radius of the pitch circle 5 of the first gear 1 is R. The second gear 2 has a geometric center 6 which coincides with its center of rotation.
If both gears were rotating about their geometric centers, the rate at which the teeth of the gears meshed and the fundamental frequency of the noise produced by this meshing would have a constant value of f.sub.0 =(N)(K.sub.1), where N is the rate of rotation of the first gear 1 in rotations per second, and K.sub.1 is the number of teeth in the first gear 1.
However, since the first gear 1 actually rotates about point 4, the rate at which the gear teeth mesh is constantly changing. A good first order approximation of the instantaneous rate at which the gear teeth mesh and the fundamental frequency of the noise produced by this meshing is given by EQU f=(N)(K.sub.1)+(N)(K.sub.1)(r)(cos(.theta.))/R
where .theta. is the instantaneous angle between the line connecting point 3 and point 4 and the line connecting point 4 and point 6. However, this equation can be reduced to the form EQU f=f.sub.0 [1+(r)(cos(.theta.))/R]
Thus, the rate of meshing of the gear teeth and the fundamental frequency of the noise produced thereby varies sinusoidally over time about a normal value f.sub.0. This phenomenon is illustrated in FIG. 2b, which is a graph of the variation over time of the fundamental frequency of the noise produced by the meshing of two gears, one of which is eccentrically mounted.
Similarly, the sound pressure level of the noise due to meshing of the gears varies sinusoidally over time. Since the rate of meshing of the gears is constantly changing, the second gear 2 is continually being accelerated or decelerated by the first gear 1. If the second gear 2 is connected to a load having great inertia, the variation in torque which must be transmitted to the second gear 2 in accelerating or decelerating it causes the sound pressure level of the noise due to meshing to vary sinusoidally about the sound pressure level which would be produced if the gears were meshing at a constant rate. This phenomenon is illustrated in FIG. 2a, which shows the sound pressure level as a function of time for one of the frequencies of noise produced by meshing of the teeth of the gears 1 and 2.
As mentioned above, the conventional method of abnormal noise detection using a frequency analyzer is unsuitable for detecting these abnormal noise phenomena. Namely, if the sampling period T of the frequency analyzer is very short (less than 1/10 of the period of rotation of the first gear 1), even if the sampling is well timed, there is no certainty of detecting the phenomena illustrated in FIG. 2. Conversely, if the sampling period T is made long, since the frequency and sound pressure level vary about constant, normal values, the variations in frequency and sound pressure level tend to be averaged out, and it is difficult to ascertain noise abnormalities from the resulting data.